Monovinylidene aromatic polymers with improved properties and a process for their preparation

ABSTRACT

The present invention is a rubber modified monovinylidene aromatic polymer comprising: 
     a) a monovinylidene aromatic polymer matrix, 
     b) rubber particles dispersed therein, characterized in that the rubber particles are produced from a diene rubber having I) a high molecular weight component and II) a low molecular weight component; the high molecular weight component having a weight average molecular weight at least two and one half times greater than the weight average molecular weight of the low molecular weight component, wherein both components I and II have a 1,4 cis content of greater than 70 percent and III) the rubber is grafted with monovinylidene aromatic polymer to the extent that there is at least 30 percent monovinylidene aromatic polymer present as grafts on the rubber.

CROSS REFERENCE STATEMENT

This application claims the benefit of U.S. Provisional Application No.60/172,103, filed Dec. 23, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to rubber-reinforced monovinylidenearomatic polymer compositions of the type commonly referred to as “highimpact polystyrene” or “HIPS”. More particularly, the invention relatesto such compositions in which the rubber-reinforcing particles areproduced from a specific polybutadiene rubber, have a specific particlesize distribution and a process for the production of such compositions.

Rubber-reinforced polymer compositions of the HIPS type are widely usedin many applications because of their ease of molding, good gloss, andgenerally good mechanical properties. It has been known for some timethat improved combinations of gloss and mechanical properties can beachieved in such rubber-reinforced polymers by providing a so-called“bimodal” distribution in the sizes of the rubber reinforcing particles,i.e. the particles show two distinct peaks in their size distribution.This can be achieved by combining two or more such resins or components,each having a group of rubber particles having a different averageparticle size. Various monovinylidene aromatic polymer compositions areknown having at least two groups of rubber particles wherein the groupshave different average particle sizes. See for example U.S. Pat. Nos.4,146,589; 4,214,056 and 4,334,039 and European Patents 0 096 447, 0 158258 and 0 152 752 which disclose such compositions.

U.S. Pat. No. 4,493,922 also discloses rubber-reinforced polystyrenecompositions having bimodal rubber particle size distributions. Theaverage rubber particle sizes disclosed for the groups of particles are2 to 8 micrometers (especially from 5 to 6 micrometers) for the group oflarger particles and 0.2 to 0.6 micrometers for the group of smallerparticles.

As mentioned, a number of methods are proposed for achieving such abimodal particle distribution. For example, U.S. Pat. No. 4,153,645discloses a method for the preparation of a HIPS-type polymer in whichtwo polymer compositions are prepared using standard productionprocesses, the compositions having different average particle sizes.These two polymer compositions are then mixed by a subsequent mechanicalblending process.

An alternative approach to producing HIPS polymers with a bimodal rubberdistribution has been to introduce feed streams of monomer and rubber attwo different points in the polymerization system. This results in apolymer product which generally has a fairly broad spread of rubberparticle sizes. Examples of this are described in U.S. Pat. No.4,334,039 and EP 0 096 447. A disadvantage of such methods is that themechanical properties of the resulting product can be somewhat poor anddifficult to control.

Yet a further approach is disclosed in U.S. Pat. No. 4,146,589 and EP 0048 389. In this method, two prepolymer compositions are preparedcontaining rubber particles with different particle sizes. Theprepolymer compositions are then mixed and further polymerized toprovide a polymer having a bimodal particle size distribution.

Other references in this area include EP-418,042 wherein the rubbercomprises a partially coupled radial or star rubber, having a ciscontent of less than or equal to about 70 percent, JP 02762722 whereinthe rubber is a mixture of a high cis polybutadiene of high molecularweight and a low cis polybutadiene of low molecular weight, and JP95005789 wherein the rubber is a mixture of a high molecular weightpolybutadiene and a low molecular weight polybutadiene, both having acis structure of greater than 80%. However, a desirable balance ofimpact strength and tensile modulus is still not attained for somespecific applications.

Therefore, it is still desirable to obtain a rubber modifiedmonovinylidene aromatic polymer having an improved balance of impact andtensile properties for select applications.

SUMMARY OF THE INVENTION

The present invention is a rubber modified monovinylidene aromaticpolymer comprising:

a) a monovinylidene aromatic polymer matrix,

b) rubber particles dispersed therein, characterized in that the rubberparticles are produced from a diene rubber having I) a high molecularweight component and II) a low molecular weight component; the highmolecular weight component having a weight average molecular weight atleast two and one half times greater than the weight average molecularweight of the low molecular weight component, wherein both components Iand II have a 1,4 cis content of greater than 70 percent and III) therubber is grafted with monovinylidene aromatic polymer to the extentthat there is at least 30 percent monovinylidene aromatic polymerpresent as grafts on the rubber.

In a preferred embodiment the present invention is a rubber modifiedmonovinylidene aromatic polymer comprising:

a) a monovinylidene aromatic polymer matrix,

b) rubber particles dispersed therein in the form of small and largeparticles, wherein the volume average particle diameter of the smallparticles is from about 0.1 to about 2 micrometers and the volumeaverage particle diameter of the large particles is from about 2 toabout 6 micrometers, characterized in that the rubber particles areproduced from a diene rubber having I) a high molecular weight componentand II) a low molecular weight component; the high molecular weightcomponent having a weight average molecular weight at least two and onehalf times greater than the weight average molecular weight of the lowmolecular weight component, wherein both components I and II have a 1,4cis content of greater than 70 percent and III) the rubber is graftedwith monovinylidene aromatic polymer to the extent that there is atleast 30 percent monovinylidene aromatic polymer present as grafts onthe rubber.

Another aspect of the present invention is a process for preparing arubber-modified monovinylidene aromatic polymer comprising the steps of:(a) continuously supplying a reaction mixture comprising monovinylidenearomatic monomer and a dissolved diene rubber to a reactor means, (b)continuously polymerizing the monovinylidene aromatic monomer in thepresence of the dissolved diene rubber in the reactor means underconditions whereby phase inversion subsequently occurs, (c) continuouslyremoving from the reactor means a diene rubber-reinforced monovinylidenearomatic polymer, which process is characterized in that: (d) the dienerubber which is dissolved in the reaction mixture has distinct high andlow molecular weight components, the high molecular weight componenthaving a weight average molecular weight at least two and one half timesgreater than the weight average molecular weight of the low molecularweight component, both components having a 1,4 cis content of greaterthan 70 percent.

In a preferred embodiment, the process further comprises (e) wherein theprocess conditions prior to phase inversion are adjusted to producedifferent groups of diene rubber particles from the high and lowmolecular weight components of the diene rubber, each group having adifferent average rubber particle size.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Monovinylidene aromatic polymers suitable for the present invention arethose produced by polymerizing a vinyl aromatic monomer. Vinyl aromaticmonomers include, but are not limited to those described in U.S. Pat.Nos. 4,666,987, 4,572,819 and 4,585,825, which are herein incorporatedby reference. Preferably, the monomer is of the formula:

wherein R is hydrogen or methyl, Ar is an aromatic ring structure havingfrom 1 to 3 aromatic rings with or without alkyl, halo, or haloalkylsubstitution, wherein any alkyl group contains 1 to 6 carbon atoms andhaloalkyl refers to a halo substituted alkyl group. Preferably, Ar isphenyl or alkylphenyl, wherein alkylphenyl refers to an alkylsubstituted phenyl group, with phenyl being most preferred. Typicalvinyl aromatic monomers which can be used include: styrene,alpha-methylstyrene, all isomers of vinyl toluene, especiallyparavinyltoluene, all isomers of ethyl styrene, propyl styrene, vinylbiphenyl, vinyl naphthalene, vinyl anthracene and the like, and mixturesthereof. The vinyl aromatic monomers may also be combined with othercopolymerizable monomers. Examples of such monomers include, but are notlimited to acrylic monomers such as acrylonitrile, methacrylonitrile,methacrylic acid, methyl methacrylate, acrylic acid, and methylacrylate; maleimide, phenylmaleimide, and maleic anhydride. It ispreferred, however, for the practice of the present invention to useeither less than about 10 percent by weight or substantially no othercopolymerizable monomer. The specified particle size distribution is nolonger optimal for larger amounts of one or more of these monomers. Ingeneral, the preferred monovinylidene aromatic monomers are styrene,alpha methyl styrene, one or more of the vinyl toluene isomers, and/ormixtures of two or more of these, with styrene being the most preferredmonovinylidene aromatic compound.

The monovinylidene aromatic polymer typically has a weight averagemolecular weight (Mw) of from approximately 120,000 to 500,000.

The rubbers preferably employed in the practice of the present inventionare those polymers and copolymers which exhibit a second ordertransition temperature which is not higher than about 0° C., preferablynot higher than about −200° C. and more preferably not higher than about−400° C. as determined or approximated using conventional techniques,e.g., ASTM Test Method D-746-52 T.

Highly preferred rubbers are alkadiene polymers. Suitable alkadienes are1,3-conjugated dienes such as butadiene, isoprene, chloroprene orpiperylene. Most preferred are homopolymers (excepting any couplingmonomers) prepared from 1,3-conjugated dienes, with such homopolymers of1,3-butadiene being especially preferred. Alkadiene copolymer rubberscontaining small amounts, for example less than 15, preferably less than10 weight percent, of other monomers such as monovinylidene aromaticscan also be employed if the rubbers meet the other qualificationsdescribed herein. The most preferred rubbers are the radial or starhomopolymers of 1,3-butadiene which have a cis content of at least 70percent and a high molecular component Mw of from about 100,000 to about1,000,000.

Preferably the rubber material has a relatively high total averagemolecular weight and a relatively low total solution viscosity andMooney viscosity, including both the high and low molecular weightcomponents. In general, the solution viscosity for the rubbery materialswill be desirably below about 300 centipoise while the Mooney viscositywill be less than about 150 (ML 4+1). As known by those skilled in theart, various techniques such as control of the branching and molecularweight control can be used to adjust and tailor these polymers toachieve the optimum solution and Mooney viscosities. The solutionviscosity of a radial-type alkadiene polymer will generally be less thana linear polymer or copolymer of the same monomeric components and samemolecular weights.

Regarding the rubber materials suitable for use according to the presentinvention, the essential requirement for the rubber material is that ithas a relatively high molecular weight component and a relatively lowmolecular weight component, wherein both components have a 1,4 ciscontent of at least 70 percent. Suitable rubbers for use herein are thepartially coupled rubbers, also called radial or star rubbers,completely coupled rubbers as well as randomly branched rubbers, otherbranched polymers and blends of rubbers, such as a blend of linear andbranched polymers, meeting the requirements for rubber materials to beemployed in this invention. The molecules of these rubber materials havethree or more polymer segments coupled by a single polyfunctionalelement or compound. Radial or star polymers having this designedbranching are conventionally prepared using a polyfunctional couplingagent. Methods for preparing star or radial polymers having designedbranching are well-known in the art. Methods for preparing a polymer ofbutadiene of this type using a coupling agent are illustrated in U.S.Pat. Nos. 4,183,877, 4,340,690, 4,340,691 and 3,668,162 and JapanesePatent 59-24 711.

Radial or star polymers, and preferably those where the “arm” segmentshave been partially coupled with tin-based, silicon-based orpolyfunctional organic coupling agents, are most preferably employed inthe practice of the present invention. The cis content of the star orradial rubbers will advantageously be greater than about 70 percent,preferably greater than 75 percent and most preferably greater than 80percent as determined by conventional IR spectrometry techniques.

These radial-type polymers have components of relatively high andrelatively low molecular weight polymer and, for this reason, typicallyexhibit two or more molecular weight peaks when molecular weight isplotted on the x-axis against weight percent of the rubber material onthe y-axis (i.e., a bimodal molecular weight distribution). As would beexpected, one molecular weight peak (the lower molecular weightcomponent) indicates the molecular weight of the uncoupled segments orlinear segments or branched polymers with lower molecular weight and asecond molecular weight peak (the higher molecular weight component)indicates the molecular weight of the coupled or branched polymer aswell as high molecular weight linear polymers.

As used herein, the molecular weights referred to are the weight averagemolecular weights or Mw's for the rubber components as determined by thegel permeation chromatographic techniques described by ASTM Test Methoddesignated D-3536 (polystyrene standard) and expressed withoutcorrection for the differences between rubber and polystyrene standards.

The polymers suitable for use according to the present inventionadvantageously exhibit a low molecular weight component Mw of at leastabout 60,000, more preferably at least about 70,000, most preferably atleast about 80,000. The ratio of the Mw of the high molecular weightcomponent to the Mw of the lower molecular weight component indicatesthe degree of coupling. In general, such ratio should be least about2.5, advantageously from about 3 to about 10, preferably from about 3 toabout 9, and most preferably from about 3 to about 8.

In addition, in order to obtain the proper proportions of the small andlarge rubber particles, it is preferred if the low molecular weightcomponent of the rubber material constitutes from about 20 to about 80weight percent of the total rubber content of the resin, preferably fromabout 30 to about 70 weight percent. Most preferably neither componentis more than about 80 percent of the total rubber in the composition.

Advantageously, to prepare a rubber-reinforced resin using mass ormass/suspension polymerization techniques, the solution viscosity of therubber of the radial-type alkadiene polymer, as measured as a 5 weightpercent solution in styrene, will be greater than 40 and less than 400centipoise (cps) at 25° C. when the viscosity is measured using aCanon-Fenske capillary viscometer (Capillary No. 400, 1.92 mm insidediameter). The minimum solution viscosity of the rubber is notparticularly critical to the practice of the invention. In a preferredrange the solution viscosity of the rubber is at least 65 and mostpreferably at least about 70 centipoise.

The Mooney viscosity values of the radial-type rubbers should be lessthan about 65, preferably less than about 90 as measured by DIN 53523.In general, to have a rubber which is sufficiently solid to be handledand processed in a normal fashion, the Mooney viscosity value should beat least about 30 and values of at least about 40 are preferred. Thepreferred range for the Mooney value is between about 20 and about 90,more preferably between about 30 and about 85, most preferably betweenabout 40 and about 80.

Although the rubber may contain a small amount of a crosslinking agent,excessive crosslinking can result in loss of the rubbery characteristicsand/or render the rubber insoluble in the monomer.

The rubber is advantageously employed in amounts such that therubber-reinforced polymer product contains from about 2 to about 20percent, preferably from about 3 to about 17 percent, more preferablyabout 3 to about 15 weight percent rubber or rubber equivalent, based onthe total weight of the rubber modified monovinylidene aromatic polymer.

The term “rubber” or “rubber equivalent” as used herein to indicateweight amounts of rubber material is intended to mean, for a rubberhomopolymer (such as polybutadiene), simply the amount of rubber, andfor a block copolymer, the amount of the copolymer made up frommonomer(s) which, when homopolymerized form a rubbery polymer. Forexample, for calculating the amount of rubber in a composition where abutadiene-styrene block copolymer rubber has been employed, the “rubber”or “rubber equivalent” of the composition is calculated based on onlythe butadiene component in the block copolymer. Obviously where physicalproperties or other aspects of the rubber material are measured, thecomplete rubber material including any comonomers is referred to.

The product of the present invention can be viewed as having a generallybroadened rubber particle size distribution. In one embodiment thepresent invention has a bimodal rubber particle size distribution with acritical amount of large and small rubber particles. The presentinvention having such distributions results in a resin product which, inthe form of molded articles, possesses improved combinations of impactresistance, tensile strength and surface gloss.

In one embodiment, according to this invention, it has surprisingly beendiscovered that products having a rubber particle size distribution ofthis type have better combinations of properties when, based on 100parts by weight rubber or rubber equivalent, (a) particles constitutingfrom about 20 to about 60 parts by weight of the rubber have diametersof from about 0.1 to about 2 micrometers, (b) particles constitutingfrom about 60 to about 20 parts by weight of the rubber have diametersof from about 2 to about 8 micrometers.

In terms of a bimodal distribution, it is found that as groups ofparticles, the group of smaller particles should have a volume averageparticle diameter of from about 0.2 to about 2 micrometers, preferablyto about 1.8 micrometers and most preferably to about 1.5 micrometersand the group of larger particles should have a volume average particlediameter of from about 2.0, preferably from about 2.5 to about 5micrometers.

As used herein, the said particle size is the diameter of the rubberparticles as measured in the resultant product, including all occlusionsof matrix polymer within rubber particles, which occlusions aregenerally present in the disperse rubber particles of arubber-reinforced polymer prepared using mass polymerization techniques.Rubber particle morphologies, sizes and distributions may be determinedusing conventional techniques such as (for larger particles) using aCoulter Counter (Coulter Counter is a Trade Mark) or, particularly forsmaller particles, transmission electron microscopy.

Regarding morphology of the rubber particles in the different groups, asis well known, the smaller particles typically have a core-shell(single, major occlusion) or cellular (multiple, minor occlusions)morphology. The larger particles would generally have a cellular orsimilar multiple-occlusion morphology.

The process of the present invention is characterized by the utilizationof a rubber having specific high and low molecular weight componentsunder process conditions whereby the above-specified rubber particlesize distribution can be obtained with standard polymerization processesand equipment.

In the preparation of the rubber-reinforced polymers, a reaction mixtureis prepared by dissolving the rubber in the monomer(s) and the resultingmonomer/rubber solution, referred to herein as the reaction mixture, issupplied to a reactor means and subsequently polymerized. The amountrubber initially dissolved in the reaction mixture is dependent on thedesired concentration of rubber in the final rubber-reinforced polymerproduct, the degree of conversion during polymerization and theviscosity of the reaction mixture solution. Specifically, the viscosityof the reaction mixture solution is advantageously less than about 3000centipoise. At higher viscosities, the reaction mixture solution isdifficult to process. Provided the viscosity of the reaction mixture isnot undesirably high, the reaction mixture solution will generallycomprise from about 5 to about 15, weight percent of the rubber, saidweight percent being based on the total amounts of rubber and monomersemployed.

Optionally, the reaction mixture will contain an organic liquid diluent.Organic liquid diluents suitably employed are normally liquid organicmaterials which do not boil at the polymerization conditions employedand which form a solution with the polymerizable monomer(3) and thepolymer prepared therefrom. Representative organic liquid diluentsinclude aromatic (and inertly substituted aromatic) hydrocarbons such astoluene, benzene, ethylbenzene and xylene; saturated or inertlysubstituted, saturated aliphatics having either straight or branchedchains of five or more carbon atoms such as heptane, hexane and octane;alicyclic or inertly substituted alicyclic hydrocarbons having five orsix carbon atoms such as cyclohexane. Preferred of such organic liquiddiluents are the inertly substituted aromatics, with ethylbenzene andxylene being most preferred. In general, the organic liquid is employedin amounts sufficient to improve the processability and heat transferduring polymerization, e.g., flow characteristics of the polymerizationmixture. Such amounts will vary depending on the rubber, monomer anddiluent employed, the process equipment and the desired degree ofpolymerization. In general, if employed, the reaction mixture willnormally contain from about 2 to about 30 weight percent of the diluentbased on the total weight of the rubber, monomer and diluent.

During the polymerization of the resulting reaction mixture, thepolymerization conditions are maintained such that phase inversionsubsequently occurs. Under such conditions the monomer is polymerizedboth with the rubber (grafted) and separately (free polymer), whichdissolved rubber thereby becomes grafted with a portion of polymerizedmonomer. The balance of free polymer, basically incompatible with therubber, forms a discontinuous smaller volume polymer/monomer phasedispersed throughout the larger volume continuous phase of themonomer/rubber (including grafted rubber) solution.

Eventually, at a point after sufficient amounts of free polymer areformed, the free polymer converts from a discontinuous phase dispersedin the continuous phase of the unpolymerized monomer(s), through a pointwhere there is no distinct continuous or discontinuous phases in thepolymerization mixture, to a continuous polymer phase having the rubberdispersed as discrete particles there through. As the polymer/monomerphase becomes the larger volume phase and hence the continuous phase,the grafted rubber forms a discontinuous phase. This is the point in thepolymerization when phase inversion occurs and the rubber becomesdispersed in the form of particles through the continuous polymer phaseresulting in a product having rubber particles dispersed in a matrix ofmonovinylidene aromatic polymer.

Preferably, at phase inversion, the rubber is sufficiently grafted suchthat the disperse rubber particles, following initial sizing, arecapable of retaining essentially the same average particle size andmorphological properties throughout the remainder of the polymerizationprocess. In a preferred embodiment, the amount of grafted rubber is atleast 30 percent of the total rubber at phase inversion. The degree ofgrafting of the diene rubber at the point of phase inversion has asignificant impact on the properties of the resultant rubber modifiedpolymer produced. Generally, the number of grafts per chain contributesto the particle size, the structure and the amount of occluded matrixpolymer within the rubber particles. Higher grafting levels results inhigher gel content, a larger amount of grafted rubber and higher graftand occluded matrix polymer content within the rubber particles. Theincreased gel content is highly desirable in that it enables anincreased rubber phase volume to be achieved. Increased rubber phasevolume can also be achieved by using a high molecular weightpolybutadiene rubber, however increasing the molecular weight typicallyincreases the solution viscosity as well, making handling and processingmore difficult. This can be avoided by producing a high molecular weightrubber which is branched and has a solution viscosity lower than alinear rubber of the same Mw. Grafting is improved due to the existenceof more diene units per rubber molecule. For example, if a rubber has amolecular weight of 200,000 and contains one graft per chain under aspecific set of process conditions, doubling the molecular weight willalso double the number of grafts per molecule.

When preparing bimodal compositions in the practice of the presentinvention the polymerization process should be conducted at conditionssuch that at the point of phase inversion, the high and low molecularweight components of the rubber form separate groups of rubber particleshaving different average particle sizes.

The polymerization process features that are utilized to achieve therequisite rubber particle distribution include the use of a graftpromoting chemical initiator, such as the peroxide initiators includingthe peresters, e.g., tertiary butyl peroxybenzoate, tertiary butylperoxyacetate, dibenzoyl peroxide, and dilauroyl peroxide, theperketals, e.g., 1,1-bis tertiary butyl peroxycyclohexane, 1,1-bistertiary butyl peroxy-3,3,5-trimethyl cyclohexane, and di-cumylperoxide, and the percarbonates; photo chemical initiation techniques;and the like. Preferred initiators include tertiary butyl peroxybenzoate, 1,1-bis tertiary butyl peroxy cyclohexane 1,1-bis tertiarybutyl peroxy-3,3,5 trimethyl cyclohexane and tertiary butyl peroxyacetate.

Initiators may be employed in a range of concentrations dependent on avariety of factors including the specific initiator employed, thedesired levels of polymer grafting and the conditions at which the masspolymerization is conducted. Specifically, in the preferred masspolymerization process for preparing rubber-reinforced polymers, fromabout 50 to about 2000, preferably from about 100 to about 1500, weightparts of the initiator are employed per million weight parts of monomerresulting in a product having rubber particles dispersed in a matrix ofmonovinylidene aromatic polymer.

With such an initiator the grafting onto the high molecular weightcomponent of the rubber is promoted and multiple grafts are formed. Thisstabilizes these rubber molecules in the reaction mixture andfacilitates the separation of the high molecular weight rubber moleculesfrom the lower molecular weight rubber. This contributes to theformation of the larger rubber particles.

The lower molecular weight component of the rubber, on the other hand,being less grafted, tends to form the smaller particles somewhat laterthan the larger particles are formed. It is also desirable to facilitatethe formation of separate, smaller particles to provide an increasedamount of agitation to the reaction mixture during and well after thepoint of phase inversion. Phase inversion has usually taken place at apoint in the polymerization process where the reaction mixture containsa solids level which, in weight percent based on reaction mixture, isabout 2.5 or about 3 times the weight content of the added rubbermaterial. Therefore, a relatively high agitation level is preferablymaintained until a point in the polymerization process where thereaction mixture contains a solids level which, in weight percent basedon reaction mixture, is at least about 3, preferably about 4 times theweight content of the added rubber material.

For example, when there is about 5 to about 10 weight percent rubberadded to the reaction mixture, relatively high agitation is maintaineduntil the reaction mixture contains about 30 percent by weight solids.As used herein, the term solids refers to the polymeric components ofthe reaction mixture such as the rubber which was added initially andthe monovinylidene aromatic polymer which has been formed.

Depending upon particular production equipment there may also be otherprocess features that can be utilized to further facilitate theformation of the specified rubber particle size distribution.

In general, continuous methods are employed for mass polymerizing themonovinylidene aromatic compound in the reaction mixture. In thepractice of the present invention it is generally preferred to utilize astratified, linear flow, stirred tower type reactor, also referred to asa plug flow type reactor. Such reactors are well known. See, for exampleU.S. Pat. No. 2,727,884. Such a process may or may not compriserecirculation of a portion of the partially polymerized product. It hasbeen determined that their utilization to prepare the product accordingto the present invention in a process of the kind described can providevery substantial improvements in the production process and in themechanical properties of the product, and in particular in productimpact resistance.

An important aspect of such polymerization processes is that asignificant portion of the polymerization of the monovinylidene aromaticmonomer in the reaction mixture can take place in the presence of thedissolved rubber. Phase inversion and precipitation and dispersion ofthe rubber particles does not occur until after sufficient grafting ofmonovinylidene aromatic polymer onto the rubber, which primarily takesplace when the rubber is in solution.

This is a major advantage over polymerization in completely mixed,stirred tank type reactors (non-stratified, non-plug flow) which areoperated at a predetermined level of conversion. Typically, due to thepresence of significant levels of the already polymerized monovinylidenearomatic polymer, the rubber which may be dissolved in the feed streamto such a reactor, is immediately dispersed as particles before graftpolymerization can occur and, more importantly, before the high and lowmolecular weight components of an appropriate rubber material canseparate and form separate groups of rubber particles.

Moreover, the process according to the present invention can bepracticed advantageously on standard mass polymerization processequipment not otherwise capable of the preparation of monovinylidenearomatic polymers with bimodal rubber particle distributions withoutsignificant equipment modifications. Such standard equipment typicallyutilizes a single supply of the unpolymerized reaction mixturecomprising a solution of rubber, monomer, optional diluent and otheradditives. The reaction mixture is then polymerized as it proceedsthrough one or a series of such reactor vessels. At the end of thereactor vessel (series) the product is removed and diluent and anyresidual monomer removed.

The polymerization mixture may also contain other additive materialsand/or polymerization aids such as plasticizers or lubricants such asmineral oil, butyl stearate or diethyl phthalate; stabilizers includingantioxidants (e.g., alkylated phenols such as di-tert-butyl-p-cresol orphosphates such as trisnonyl phenyl phosphate); chain transfer agent,such as an alkyl mercaptan such as n-dodecyl mercaptan; or mold releaseagents, e.g., zinc stearate; all of which additives and/orpolymerization aids are added to the reaction mixture where appropriateincluding before, during or after polymerization.

The use of a chain transfer agent is optional and is usually employedonly in the production of a composition or prepolymer containing largersize rubber particles (e.g. having an average particle size of at leastone micrometer). If employed, the chain transfer agent is generallyemployed in an amount of from about 0.001 to about 0.5 weight percentbased on the total weight of the polymerization mixture to which it isadded.

The temperatures at which polymerization is most advantageouslyconducted are dependent on the specific components, particularlyinitiator, employed but will generally vary from about 60 to about 190°C.

Crosslinking of the rubber in the resulting product and removal of theunreacted monomers, as well as any reaction diluent, if employed, andother volatile materials is advantageously conducted employingconventional techniques.

The rubber modified monovinylidene aromatic polymer of the presentinvention has an excellent balance of impact and tensile modulusproperties, allowing for its use in various applications includinginjection molding applications, extrusion applications, foamapplications including large appliances, consumer electronics, airconditioners, refrigerators, freezers, small appliances, cassettes,radio, TV, stereo cabinets, furniture and furnishings, toys, housewares,building and construction applications, footwear, medical applications,packaging, disposables such as tumblers, glasses, dishes, cups, bowls,flatware, cutlery, blowmolded items, foam board, sheet, films and thelike.

What is claimed is:
 1. A rubber modified monovinylidene aromatic polymercomprising: a) a monovinylidene aromatic polymer matrix, b) rubberparticles dispersed therein, characterized in that the rubber particlesare produced from a diene rubber having I) a high molecular weightcomponent and II) a low molecular weight component; the high molecularweight component having a weight average molecular weight at least twoand one half times greater than the weight average molecular weight ofthe low molecular weight component and the low molecular weightcomponent constitutes from about 20 to about 80 weight percent of thetotal rubber content, wherein both components I and II have a 1,4 ciscontent of greater than 70 percent and III) the rubber is grafted usinga graft promoting chemical initiator, with monovinylidene aromaticpolymer to the extent that the amount of grafted rubber is at least 30percent of the total rubber at phase inversion.
 2. The rubber modifiedmonovinylidene aromatic polymer of claim 1, wherein the rubber particlesare dispersed in the form of small and large particles, wherein thevolume average particle diameter of the small particles is from about0.1 to about 2 micrometers and the volume average particle diameter ofthe large particles is from about 2 to about 6 micrometers and the smallrubber particles are from 20 to 80 weight percent of the total rubber.3. The rubber modified monovinylidene aromatic polymer of claim 2,wherein the small particles have a volume average particle diameter offrom 0.1 micrometers to 1.8 micrometers.
 4. The rubber modifiedmonovinylidene aromatic polymer of claim 3, wherein the small particleshave a volume average particle diameter of from 0.1 micrometers to 1.5micrometers.
 5. The rubber modified monovinylidene aromatic polymer ofclaim 1 wherein the monovinylidene aromatic polymer is polystyrene. 6.The rubber modified monovinylidene aromatic polymer of claim 1 whereinthe rubber is polybutadiene.
 7. The rubber modified monovinylidenearomatic polymer of claim 1, wherein the diene rubber is branched. 8.The rubber modified monovinylidene aromatic polymer of claim 1 whereinthe diene rubber has a 1,4 cis content of greater than 80 percent. 9.The rubber modified monovinylidene aromatic polymer of claim 1 whereinthe low Mw component has a Mw of at least 60,000.
 10. The rubbermodified monovinylidene aromatic polymer of claim 1 wherein (a)particles constituting from about 20 to about 60 parts by weight of therubber have diameters of from about 0.1 to about 2 micrometers, (b)particles constituting from about 60 to about 20 parts by weight of therubber have diameters of from about 2 to about 6 micrometers.
 11. Therubber modified monovinylidene aromatic polymer of claim 1 wherein (a)particles constituting from about 30 to about 50 parts by weight of therubber have diameters of from about 0.2 to about 2 micrometers, (b)particles constituting from about 50 to about 30 parts by weight of therubber have diameters of from about 2 to about 6 micrometers.
 12. Anarticle produced from the rubber modified polymer of claim 1.